Method to characterize curing of epoxy with aromatic diamines

ABSTRACT

A method for determining the extent of polymerization of an epoxy system consisting of adding a fluorescent label, the label having a reactivity similar to the reactivity of the curing agent, to the epoxy system and measuring the fluorescence intensity during polymerization. The fluorescence intensity of the label at its emission maximum increases sharply with polymerization, allowing one to follow cure reactions in a quantitative way by measuring fluorescence, especially in the later stages of cure beyond gelation. The composition, reactivity ratio, activation energy and weight-average molecular weight can also be determined by adding a label, such as p,p&#39;-diaminoazobenzene (DAA) or trans diamine-stilbene (DAS), to mimic the reactivity of the curing agent, such as diaminodiphenyl sulfone (DDS) or methylene dianiline (MDA), and measuring the UV-VIS spectra in addition to the fluorescence intensity. Reaction of the label with the epoxide produces spectrophotometric shifts which are deconvoluted based on the band assignments of model compounds to obtain compositional analyses of cure products.

The Government has rights in this invention by virtue of Grant No. DMR-82-05897 from the National Science Foundation, Polymers Program, Contract No. DL-H-225243 from C. S. Draper Laboratories, and Contract No. DAAG29-85-K-0055 from the Army Research Office.

This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 800,443 filed Nov. 21, 1985 by Chong Sook Paik Sung and In-Joo Chin entitled "Method to Characterize Curing of Epoxy with Aromatic Diamines by Azochromophore Labelling" now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to a method of chemical analysis and particularly to a method for characterizing the curing of an epoxy.

The structure and properties of epoxies are known to strongly depend on the extent of cure and physical aging which has taken place after the cure cycle is completed. A number of physicochemical techniques have been used or developed toward a better characterization of cure and physical aging phenomena in epoxies. Among them are such techniques as FT-IR spectroscopy, described by M. K. Antoon et al. in Polym. Comp., 2, 81 (1981); thermal analyses described by G. L. Hagnauer et al. in ASC Symp. Ser., 221, 229 (1983); GPC (size exclusion chromatography), described by G. L. Hagnauer et al. in ASC Symp. Ser., 227, 25 (1983) and ASC Symp. Ser., 221, 193 (1983); microdielectrometry, described by N. F. Sheppard et al. in Proc. of the 26th SAMPE Symposium, Los Angeles, 65, (1981); torsional braid analyses described by J. B. Enns et al. in ACS Symp. Ser., 203, 27-63 (1983); ¹³ C solid-state NMR, described by A. N. Garroway et al. in Macromolecules, 15, 1051 (1982); thermally stimulated current measuement, described by T. D. Chang et al. in Polym. Eng. Sci., 22, 1213 (1982); fluorescence, described by R. L. Levy et al. in ACS Org. Coat. Appl. Polym. Sci. Proc., 48, 116 (1983) and Wang et al. in ACS Polym. Mater. Sci. Eng. Proc., 49, 138 (1983); and ESR spectroscopy, described by A. Gupta et al. in J. Appl. Polym. Sci., 28, 1011 (1983).

While these experimental techniques provide useful information on the extent of cure and on epoxy structure, there are certain limitations and disadvantages associated with each technique. For example, FT-IR fails to monitor later stages of cure when the epoxy peak disappears. The use of GPC and of size exclusion chromatography is limited to the early stages of the curing reaction. The fluorescence techniques measure the emission of the epoxy as a function of increasing viscosity, not the formation of reaction products. ESR techniques also primarily measure decreasing mobilities of the label with increasing cure and viscosity but provide only limited information on the reaction products. For example, Brown and Sandreczki in Macromolecules, 16 1980 (1983) observed different ESR spectra which may allow spectroscopic monitoring of the initial addition products of the reaction of an epoxy with a nitroxide monoamine, if the reactivity of the nitroxide is similar to that of a diamine.

¹³ C MAS-CP (magic angle spinning-cross polarized) solid-state NMR is another technique for the characterization of polymeric solids. As Garroway et al. demonstrated, it can provide information on certain molecular motions in epoxy. It is not easy, however, to obtain quantitative compositional information on the reaction products by ¹³ C NMR in cross-linked polymers due to such complications as line broadening and spinning side bands. The fact that the peak intensity is not generally representative of the concentration in crosspolarization experiments is another problem. ¹⁵ N NMR, in principle, may be more useful, but its sensitivity is poor because of the low natural abundance.

The article entitled "A New Method to Characterize Curing of Epoxy with Aromatic Diamines by Azochromophore Labeling" by In-Joo Chin and Chong Sook Paik Sung in Macromolecules 12, 2603-2607 (1984) describes a new method to obtain quantitative compositional information on the curing of epoxy with aromatic diamines by azochromophore labelling. In this technique, a small amount of an azochromophore, such as p,p'-diaminoazobenzene (DAA), which has reactivity similar to the curing agent, such as diaminodiphenyl sulfone (DDS), is used to provide an indication of the extent of cure. As the epoxy is cured, λ_(max) of the π→π* transition corresponding to the azo bond of the curing agent is red-shifted in a way that provides spectral discrimination for the cure products (cross-linkers, branch points, linear chains, chain ends and unreacted diamines). The accuracy of the compositional analyses depends on the proper assignments of λ_(max) positions and the determination of the extinction coefficients for the various cure products.

Epoxies formed with aromatic diamines such as DGEBA-DDS (diglycidyl ether of bisphenol A-diaminodiphenyl sulfone) are important for high-temperature applications. In the DGEBA-DDS system, at the stoichiometric ratio, there are five major species during curing. This is because the amine groups are the major reactive species, and the hydroxyl groups do not initiate epoxy-polymerization in uncatalyzed epoxy cured with aromatic diamines at the stoichiometric ratio.

In order to understand cure mechanisms, kinetics, and structure/property correlations, it is important to know the relative concentrations of the primary reaction products during the course of the curing process.

It is therefore an object of the present invention to provide a means for determining the extent of cure in applications utilizing epoxies of varying thickness and in particular on metallic substrates in situ.

It is another object of the present invention to provide a process for determining the relative concentrations of the primary reaction products produced during the curing of epoxies.

It is yet another object of the invention to provide a sensitive method for determining the reactivity ratios, the activation energy, and the weight-average molecular weight of the reaction products produced during the polymerization of the epoxy.

SUMMARY OF THE INVENTION

The present invention is a method, and labelled epoxy system for use in the method, for monitoring polymerization of the epoxy system. The invention is based on the discovery that epoxy labelled with an azobenzene dye, an azonaphthalene dye, a benzylidene-dianiline dye or a conjugated stilbene dye exhibits very sensitive changes in fluorescence intensity corresponding to emission by the label as a function of cure extent. The label should be selected to have a reactivity similar to that of the curing agent. As shown by the following examples using three different epoxy systems, fluorescence intensity at the emission maximum of the label increases sharply after gelation, allowing for sensitive cure monitoring during the later stages of cure.

By combining this method with the method of monitoring the relative composition of cure products utilizing the deconvolution of the UV-VIS spectra during polymerization of the epoxy system, parameters characteristic of cure kinetics and mechanisms such as cure product composition, reactivity ratio of primary amine versus secondary amine, initial rate constants and activation energy may be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 are the chemical structures of the major model compounds expected from the reaction of DAA and GPE at 145° C. under N₂ with a varying excess of GPE.

FIG. 2 is the UV-VIS spectra of p,p'-diaminoazobenzene (DAA) in a stoichiometric mixture of DGEBA-DDS (FIG. 2a) or DGEB-DDS (FIG. 2b) as a function of cure time at 160° C.

FIG. 3 is the kinetic scheme of the epoxy cure reactions, including the various cure products.

FIG. 4 is a graph of the theoretical composition of cure species as a function of amine reaction (εa), according to equations 8-12 for reactivity ratios of 0.1 (FIG. 4a) and 2 (FIG. 4b).

FIG. 5 are graphs of the experimental composition of cure species as a function of extent of amine reaction for DGEBA-DDS (FIG. 5a) and DGEB-DDS (FIG. 5b), combining data from several cure temperatures. ( . . . . . ) represents predicted curve for a reactivity ratio of 0.5. ( - - - - ) represents predicted curve for a reactivity ratio of 2.

FIG. 6 is the fluorescence spectra of p,p'-diaminoazobenzene in a stoichiometric mixture of DGEBA-DDS (FIG. 6a) and DGEB-DDS (FIG. 6b) as a function of cure time at 160° C. (excitation at 456 nm).

FIG. 7 is a comparison of the cure compositions by UV-VIS deconvolution and by fluorescence intensity.

FIG. 8 is a graph of I_(f) versus ε_(a) correlating relative fluorescence at 565 nm with the extent of amine reaction by UV-VIS.

FIG. 9 is a comparison of the predicted weight soluble fraction as a function of the extent of amine reaction with composition of cure products.

FIG. 10 is the UV-VIS absorption spectra (FIG. 10a) and fluorescence spectra (FIG. 10b) of DAS in DGEB-MDA epoxy as a function of cure time at 140° C.

FIG. 11 is the fluorescence intensity at 418 nm as a function of cure time at 140° C. and 120° C. in DAS labelled DGEB-MDA epoxy.

DETAILED DESCRIPTION OF THE INVENTION

Fluorescence due to a chromophore label in an epoxy system increases sharply due to the increasing fluorescence quantum yield of cure products containing tertiary amines. Fluorescence intensity is therefore used to determine the extent of cure, as well as to estimate composition, based on kinetic differential equations. UV-VIS spectral deconvolution can also be used to determine cure extent.

The method of determining the extent of curing of an epoxy system by adding a chromophore label to the system and measuring the fluorescence intensity is applicable to any multifunctional epoxy system. The chromophore label should be selected to have a reactivity similar to the reactivity of the epoxy curing agent. Fluorescence is determined at the emission maximum of the chromophore label.

In examples utilizing DGEBA-DDS, DGEB-DDS and DGEB-MDA epoxy systems labelled with DAA or DAS, fluorescence was used to determine relative composition of cure products, the extent of cure, or for comparison with the theoretically predicted weight average molecular weight and soluble fractions. UV-VIS and IR techniques were also used to determine the extent of cure in the epoxy systems. Analysis of the UV-VIS spectra and the fluorescence spectra allows one to determine the cure product composition, reactivity ratio of primary amine versus secondary amines, initial rate constants and activation energy. Since IR and thermal analyses suggested that DAA reacts a little faster than DDS, a calibration curve is constructed to correct for differences in the reactivities between the label and the curing agent.

Epoxy networks studied included DGEBA-DDS, DGEB-DDS, and DGEB-MDA epoxy. The first epoxy, DGEBA epoxy (diglycidyl ether of bisphenol A) has a maximum T_(g) of 215° C. which leads to a complete cure, subject to degradation. When cured below 215° C., vitrification occurs some time after gelation (e.g., 150 minutes at a cure temperature of 160° C.) according to its T-T-T diagram. Since there is little progress in the cure reaction after vitrification, the extent of cure is limited. The second epoxy, DGEB epoxy (diglycidyl ether of butane diol), has a maximum T_(g) of about 80° C. The cure reaction of DGEB can progress much further at cure temperatures above 80° C. due to the absence of vitrification.

The polymerization of these epoxy systems were characterized as follows, although such characterization is not required to practice the method of determining the extent of cure by measuring the fluorescence intensity of the chromophore label. First, the quantitative composition of each product over a wide range of cure extents were obtained for each of the epoxy networks cured at three isothermal temperatures. The reactivity ratio of the primary amine and the secondary amine with the epoxy group were then determined from this experimental data. The reactivity ratio is predicted to have a strong effect on the cure process including parameters such as gel time, molecular weight and the elastically active network chains. The activation energy is estimated from the initial slopes of the rate constants of the epoxy-primary amine reaction. The data are then compared assuming the weight average molecular weight as a function of cure using Miller and Macosko's recursive theory for network, described in Macromolecules, 13, 1063 (1980). Predicted soluble fractions are also compared with the composition of cure products.

The total fluorescence intensity by the DAA or DAS label at its emission maximum increases sharply as the cure proceeds. This behavior of DAA is not due to the viscosity change, but rather to the increasing fluorescence by the cure products. For DAS, fluorescence increase is due to viscosity as well as the cure reactions. The observed fluorescence intensities per mole of the model compounds representing each cure product in the DGEB-DAA and DGEBA-DAA epoxy system are in the following ratios when excited at 456 nm: cross-linker (1400):branch point (1100):linear chain (18):chain end (9): DAA (1). Assuming the above ratios and using the known concentration of each cure product by deconvolution of UV-VIS spectra, the observed overall fluorescence intensity can be modeled as the sum of contributions from each cure product. Thus, fluorescence can be used to monitor cure reactions in DAA labelled epoxy.

In the following examples, the extent of cure of DGEBA-DDA, DGEB-DDA and DGEB-DDS epoxy was determined by the fluorescence method. Using the ratio of the amine functionality estimated from UV-VIS data, the composition of cure products at a given fluorescence intensity value were also estimated. The results are compared with those obtained by UV-VIS spectral deconvolution, as described by the article entitled "A New Method to Characterize Curing of Epoxy with Aromatic Diamines by Azochromophore Labeling" by In-Joo Chin and Chong Sook Paik Sung in Macromolecules 12, 2603-2607 (1984).

EXAMPLE OF CHARACTERIZING THE POLYMERIZATION OF DGEBA-DDS AND DGEB-DDS LABELLED WITH DAA

1. Synthesis of Model Compounds for determination of the quantitative composition over a wide range of cure extents

p,p'-Diamino azobenzene (DAA) from Eastman Kodak Chemical, Rochester, NY was purified by passing it through basic alumina columns. This method of purification leaves little residue in a TLC plate in comparison to DAA recrystallized from toluene and acetone. Purified DAA has a melting point in the range of 238° C. to 241° C. 1,2-Epoxy-3-phenoxypropane (glycidyl phenyl ether, GPE) from Aldrich Chemical Co., Milwaukee, WI was used without further purification.

Model compounds were made by reacting DAA and GPE at 145° C. under N₂ with a varying excess of GPE. FIG. 1 shows the chemical structures of the major model compounds expected from this reaction. In order to obtain intermediate cure products, DAA and GPE were reacted in 1:4 molar ratio for 10 or 30 minutes at 145° C. After the reaction products were developed with a mixture of benzene:acetone:diethyl amine (16:3:1) five times, three well-separated spots in a TLC plate were seen. The top spot corresponds to DAA and the two lower spots (Fraction A and B) were purified and their properties summarized in Table 1.

In order to obtain products of the later stages of the reaction, DAA was reacted with a large excess of GPE (6, 8, or 30 fold excess) for periods up to 4 hrs. at 150° C. under N₂. The major reaction products (Fraction C and D) appeared between Fraction A and B after developing 4 times with a 13:1:1 mixture of benzene:acetone:diethylamine. The reaction mixture was initially fractionated by flash chromatography using silica gel at a pressure of 25 lbs/in² with the same solvent mixture described above. Out of the total fractions collected from flash chromatography, mother liquids of fractions 11 to 19 were combined after adding a 1:1 mixture of petroleum ether and ethanol to precipitate an orange powder. Preparatory TLC was used to separate Fraction C and D from this mother liquid by the same solvent mixture after developing 8 times. Their physical characteristics are also summarized in Table I.

                                      TABLE I                                      __________________________________________________________________________     Physical Characteristics of the Fractions                                      Obtained by Reaction of DAA and GPE                                                                     Nitrogen                                              Fraction                                                                            Color M.P. Mass Spec                                                                               (cal'd/obs)                                                                            UV-VIS/μm                                  __________________________________________________________________________     A    Brown <70° C.                                                                      m.w. 362 obs.                                                                           15.46/13/79                                                                            420                                           B    Reddish                                                                              185-6° C.                                                                    m.w. 512 obs.                                                                           10.93/10.75                                                                            445                                           C    Yellowish                                                                            177° C.                                                                      m.w. 662 not enough for                                                                         458                                                           (small peak) obs.                                                                       analysis                                              D    Orange                                                                               172° C.                                                                      not available                                                                           6.39/6.49                                                                              460                                           __________________________________________________________________________      *Calculated based on the assumption that the Fraction A, B, and D              correspond to the Model compounds 1, 2, (or 3) and 5, respectively.            **The main λ.sub.max is indicated while there were shoulders            present in some fractions. The extinction coefficient at the main peak wa      approximately 4 × 10.sup.4 moles/cm, independent of the fractions. 

Fractions A, B, C and D obtained from model reactions were further analyzed by reverse phase HPLC using a Waters Associate, Milford, MA analytical system interfaced with a Spectra Physics data system. Reverse phase (Bondapak C₁₈) columns were employed, using a solvent programmer gradient and a varying amount of acetonitrile in water. An ultraviolet detector set at 280 nm was used.

2. Cure composition from analysis of the UV-VIS spectra

DGEBA was recrystallized from saturated methyl ethyl ketone solution by seeding it with purified DGEBA crystals and leaving it in the freezer (-15° C.) for one to two weeks. DAA was recrystallized from toluene and acetone. DDS and DGEB were purchased from Aldrich and used without purification.

In typical cure monitoring studies, a small amount of DAA (approximately 5 to 7 mg or about 0.1% by weight for UV-VIS studies and 0.01% by weight in most fluorescence studies) was mixed with a stoichiometric mixture of DGEBA (5.0 g) or DGEB (2.98 g). DDS (1.825 g) was then added and the mixture heated with a magnetic stirrer at 120° C. for 5 minutes. Two circular quartz plates were clamped together with two thin Mylar films (1.5 mil) on the edges leaving a center space for sample. The clamped quartz plate with Mylar spacers were dipped into epoxy heated to 100° C. The sample enters the center space by capillary action.

UV-VIS spectra and fluorescence spectra were measured after curing the epoxy in an oven for a specific time and cooling the sample to room temperature. Fluorescence was measured with a 1 nm wide excitation slit and a 10 nm wide emission slit using a Perkin-Elmer MPF-66 spectrometer with a Model 7500 Data Station. UV-VIS spectra were obtained with a Perkin-Elmer Diode Array (Model 3840) System with a Model 7500 Data Station. The area under the UV-VIS peak was used to calibrate for concentration and thickness changes during cure to obtain the relative fluorescence intensity. UV-VIS spectra were deconvoluted with a computer program assuming a Gaussian distribution curve for each cure species. The λ_(max) 's of the various cure products were assigned 410, 420, 445, 460 and 470 nm, respectively.

The model compounds were analyzed as follows: Table 1 shows the physical characteristics of Model Compounds, Fractions A to D, which were purified by thin layer chromatography after reactions of DAA and GPE (in varying excess) at 145° C. under N₂. There is a distinct color change as the fractions change from A to D. Fraction A is brownish as is DAA itself, while Fraction B is reddish. Fractions C and D are yellow and orange, respectively. Results of nitrogen analyses are close to the calculated results assuming that Fraction A, B, or D corresponds to the Model compound 1, 2 or 5, respectively, as shown in FIG. 1. The UV-VIS spectrum shows shoulders in addition to the main absorption peak.

In order to ascertain if Fractions A to D are pure, reverse phase high pressure liquid chromatography using mixtures of acetonitrile and water as the mobile phase was carried out. A starting gradient of acetonitrile:water of 50:50 was used. DAA appears at 3.52 (76%) while GPE appears at 3.80. Fraction A shows a main peak at 5.72 (91%) and several small impurity peaks <2% each). Fraction B shows its main peak at 7.26 (95%). Fraction C shows four peaks between 7 to 9, with two major peaks at 7.80 (68%) and 8.48 (16%). Fraction D shows a main peak at 7.20 (81%) and a small peak at 8.44 (7%).

While Fractions A and B are relatively pure, Fractions C and D are certainly mixtures. Closely-overlapping peaks for Fractions C and D were spread out by using a different starting gradient (80/20 ratio of acetonitrile to water). After the major peak was separated, the UV-VIS scan was taken in the HPLC instrument by switching the UV-VIS detector to the scanning mode. The maximum absorption for the major peak for the Fractions C and D is about 460 nm and 470 nm, respectively. For Fractions A and B, the absorption maxima were 420 nm and 445 nm, respectively. Since Fraction B is quite pure, it could be either model compound 2 or 3. These λ_(max) positions, as summarized in Table II, were used in the deconvolution of the peaks obtained in the epoxy system. The extinction coefficient of about 4×10⁵ mole/cm was measured regardless of the fractions so that an equal extinction coefficient was assumed in the deconvolution of the curves.

                  TABLE II                                                         ______________________________________                                         Positions of λ.sub.max for Model Compounds                              Positions of DAA and GPE                                                       Model Compound    λmax (nm)                                                                         Δλ                                    ______________________________________                                         DAA/(PP)          410        0                                                 1st (PS)          420       10                                                 2nd (SS)          445       35                                                 3rd (PT)                                                                       4th (ST)          460       50                                                 5th (TT)          470       60                                                 ______________________________________                                          *refer to FIG. 1 for chemical structure. Designation in parentheses: P is      primary amine, S is secondary amine and T is tertiary amine.             

FIG. 2 compares UV-VIS spectra obtained as a function of cure time in DGEBA-DDS (FIG. 2a) and DGEB-DDS (FIG. 2b) at 160° C. In both sets of sepctra, significant red shifts are observed with increasing cure time. In DGEBA-DDS epoxy, the matrix gels after about 50 minutes of cure and vitrifies after 150 minutes of cure. The cure reaction is quenched after vitrification. As a consequence, UV-VIS spectra in DGEBA-DDS matrix does not show much change after vitrification, as shown by "e" in FIG. 2a corresponding to 300 min. cure time. In contrast, no vitrification occurs at 160° C. in DGEB-DDS epoxy since its maximum T_(g) is only about 80° C. The cure reaction for the DGEB-DDS epoxy has been pushed further, as indicated by additional red shifts shown in "e" of FIG. 2b.

In order to insure that these spectral shifts are due to cure reactions and not to the changes in the matrix (e.g., polarity change as a function of cure), UV-VIS spectra of the model compound representing cross-linkers in DGEBA-DDS epoxy were run as a function of cure at 160° C. Any spectral shift in this case would be due to the matrix change since the cross-linkers cannot react any further. Only a negligible (<5 nm) spectral shift was observed. The spectral shifts obtained in DAA labelled epoxy were therefore interpreted as arising from cure reactions only.

Table III summarizes the results on the composition of cure products by deconvolution of UV-VIS spectra with a computer program by assuming λ_(max) positions of the model cure products according to Table II and a Gaussian distribution curve for each species. The error in resolving closely overlapping peaks in the spectra can be significant, especially when the cure is intermediate, for example, as with "b" and "c" in FIGS. 2a and 2b, however, the curve is fitted until tthe overall error calculated by the program is below two percent. The error in the composition of each cure product can still be as large as several percent, since the composition, given in Table III, corresponding to a certain cure time may not be an unique solution to the particular spectrum.

The computer program used in these examples is attached as part of the specification. Similar programs for deconvolution of the spectra are commercially available.

                                      TABLE III                                    __________________________________________________________________________     Composition of Cure Products in DGEB-DDS and DGEBA-DDS                         Epoxy as a Function of Cure Time at 160° C.                                                              Extent of Amine                                       Cure Time                                                                            Cure Products (%)  Reaction                                      Epoxy   (min.)                                                                               A.sub.PP                                                                          A.sub.PS                                                                          A.sub.SS + A.sub.PT                                                                   A.sub.ST                                                                          A.sub.TT                                                                          (εa)                                  __________________________________________________________________________     DEGB-DDS                                                                                0    78 20  2      0  0 .059                                                   5    44 34 14      6  2 .219                                                  10    33 33 16     11  7 .318                                                  20    13 20 32     22 14 .510                                                  30     6 12 20     31 31 .676                                                  45     2  8 18     33 40 .750                                                  70     2  4 17     36 41 .775                                                  100    1  5 13     30 52 .815                                                  150    1  5 11     21 62 .848                                                  300    1  3 10     37 47 .814                                                  800    1  3 13     34 49 .813                                          DEGBA-DDS                                                                               0    75 25  0      0  0 .063                                                   5    61 20  9      6  5 .183                                                  10    35 41  9      9  5 .270                                                  15    25 28 36      5  5 .343                                                  30    10 17 17     23 34 .636                                                  45     8 10 23     25 33 .653                                                  60     9 15 19     25 33 .646                                                  100   10 15 26     25 23 .667                                                  150   10 15 15     29 27 .675                                                  300    8 14 23     29 27 .638                                                  140    7 20 12     20 42 .680                                          __________________________________________________________________________

The last column of Table III lists the extent of amine reaction (ε_(a)) as defined by the following equation:

    ε.sub.a =[A.sub.ps +2(A.sub.ss +A.sub.pt)+3A.sub.st +4A.sub.tt ]/4 (1)

where A_(ps), A_(ss), A_(st) or A_(tt) corresponds to the fractional amount of cure products, namely chain ends, linear chains, branch points or cross-linkers, respectively. The cure products are shown in FIG. 3. A_(pp) is the fraction of unreacted diamine.

As demonstrated by Table III: the fractions of branch points and cross-linkers increase with cure time for both epoxy matrices, as predicted by the spectral shifts of FIG. 2, and the cure reaction of DGEBA-DDS epoxy seems somewhat slower than DGEB-DDS, expecially at long cure times, as shown by the values of the cure products as well as by comparing the overall extent of amine reaction (ε_(a)) versus cure time (minutes) for both epoxy systems DGEB-DDS and DGEBA-DDS when cured at 160° C. In DGEBA-DDS, the maximum ε_(a) is only about 70%, presumably due to quenching by vitrification. Even in DGEB-DDS where vitrification does not occur at the cure temperatures, ε_(a) only reaches about 85% at this temperature.

The extent of amine cure for both epoxy systems is temperature dependent, as shown by a graph of ε_(a) versus time of curing (minutes) at 140° C., 160° C., and 180° C. At early stages of the cure, the effect of higher temperature is quite pronounced, leading to greater cure extent. However, at later stages of cure, the temperature does not have much effect, especially for DGEBA epoxy, leading to a plateau in the overall extent of reaction. This phenomenon is undoubtedly due to the vitrification occurring in DGEBA epoxy in comparison to the absence of vitrification of DGEB epoxy. The maximum ε_(a) obtained for DGEB-DDS when cured at 180° C. is about 95%.

3. Analysis of the cure kinetics and mechanisms

Since epoxy homopolymerization may be neglected in the absence of catalysts, the major cure reactions can be assumed to be the reactions between epoxide and amine groups, as shown in FIG. 3. The kinetic scheme defines the rate constant k₁ as due to the conversion of primary amine to the secondary amine while k₂ is due to the conversion of secondary amine to the tertiary amine.

Based on FIG. 3, one can write a series of kinetic differential equations, as described by Dusek et al in J. Polym. Sci., Symp. No. 53, 29 (1975), as follows: ##EQU1## where [b] is the concentration of unreacted epoxy groups.

Solving the above equations, one can obtain the fraction of each cure species only as a function of the reactivity ratio of k₂ /k₁ and the fraction of unreacted diamine (A_(pp)): ##EQU2## where r=k₂ /k₁, p=1/(1-r/2) and q=(1+r/2)2.

The overall extent of amine reaction (ε_(a)) written in terms of App and r only, is as follows:

    ε.sub.a =1-[1/(2-r)][(1-r)A.sub.pp 1/2+A.sub.pp r/4](13)

A calculated fraction of each cure product and of the unreacted diamine is plotted as a function of ε_(a), as shown in FIG. 4. The reactivity ratio, r, is assumed to be 0.1 in FIG. 4(a) and 2 in FIG. 4(b). Comparing these two figures, it becomes obvious that the smaller the r value, the greater the magnitude of A_(ps), A_(ss) and A_(st). When r=2, A_(pt) becomes noticable while it is negligible when r is less than one. At smaller value of r, A_(pp) also decreases faster while A_(tt) increases slower than at greater value of r.

The r value has a strong effect on many structural parameters of the epoxy network. The fraction of each cure product is deconvoluted from the UV-VIS spectra obtained at three cure temperatures (140°, 160° and 180° C.) and plotted as a function of the extent of amine reaction in FIGS. 5a and 5b. As shown in FIG. 5a, experimental data are only available up to about 75% reaction due to vitrification occurring in DGEBA-DDS epoxy. In contrast, experimental data are available up to 95% reaction in DGEB-DDS epoxy. Comparing the theoretical prediction of the composition of cure species as a function of amine reaction according to equations 8-12 for a reactivity ratio of 0.1 with the experimental composition of cure species as a function of extent of amine reaction, shown in FIGS. 5a and 5b, allows one to determine the best fit r, or reactivity ratio to be used to deconvolute the spectra. The theoretical curves according to the equations 8-12, corresponding to r of 0.5 or 2, are plotted. The r value which seems to describe the experimental points best, in view of the range of experimental error, is then determined. As demonstrated, this is a r value close to unity. Thus, reaction rates of primary amine-epoxy and secondary amine-epoxy are practically indistinguishable in this system.

k₁ is then calculated from the kinetic equation (2). In order to integrate Eqn. (2), [b] (concentration of unreacted epoxy) and [A_(pp) ] are expressed in terms of ε_(a).

By definition

    1-ε.sub.b =[b/4]

Rearranging, the above equation becomes

    [b]=4-4ε.sub.b                                     (14)

In a stoichiometric mixture, ε_(a) =ε_(b). For the case of r=1, Eqn. (13) is reduced to

    ε.sub.a =1-[A.sub.pp ].sup.1/4                     (15)

Rearranging Eqn. (15) leads to

    [A.sub.pp ]=(1-ε.sub.a).sup.4                      (16)

Substituting Eqns. (14) and (16) into Eqn. (2), ##EQU3## Rearrangement of Eqn. (17) leads to ##EQU4## Plotting ε_(a) /(ε_(a) -1) versus time for DGEBA-DDS epoxy at three cure temperatures demonstrates that at cure times beyond gelation, the reaction rate constant which is proportional to the slope of these curves is clearly reduced. By drawing a straight line through the first few data point for the slope, k₁ is estimated to be 5.4×10⁻³, 1.3×10⁻² and 3.1×10⁻² min⁻¹ at 140° C., 160° C., and 180° C., respectively.

An activation energy of 15.7 Kcal/mole and a pre-exponential factor of 1.2×10⁶ min⁻¹ were estimated from an Arrhenius plot combining the data for both epoxy matrices.

4. Fluorescence Studies

The inherent fluorescence of the epoxy matrix itself is not useful to monitor cure reactions, as shown by the following: When excited at around 320 nm, both DGEBA-DDS and DGEB-DDS are highly fluorescent with an emission maximum around 370 to 380 nm. However, when the fluorescence intensity is calibrated for thickness fluctuations, the fluorescence intensity is constant with the extent of cure.

In contrast, when the DAA label is excited, for example at 456 nm, a strongly cure dependent behavior of fluorescence intensity is observed. FIGS. 6a and 6b shows such fluorescence spectra for DGEBA-DDS-DAA and DGEB-DDS-DAA epoxy matrices respectively in the spectral range of 450 nm to 800 nm. In both epoxy matrices, at zero cure time, hardly any fluorescence is observed. But with increasing cure time, fluorescence with a broad emission peak around 560 nm increases. At long cure times, the emission peak seems to have red-shifted slightly, by 5 to 10 nm. This small red shift is in sharp contrast to much larger red shifts observed in UV-VIS spectra, shown in FIGS. 2a and 2b. Changes of polarity in the solvent medium is known to cause large shifts in emission spectra. Therefore, one can conclude that polarity did not change much as the epoxy cured. This trend was also suggested by small shifts in UV-VIS spectra by the model compound representing a cross-linker.

In order to quantify the fluorescence intensity changes, relative fluorescence intensity at 560 nm (after dividing by the UV-VIS peak area of the label) is plotted as a function of cure time at three cure temperatures for both epoxies. Fluorescence intensity for DGEB epoxy is about the same as for DGEBA epoxy up to gelation time. However, fluorescence increases continuously beyond gelation for DGEB but levels off in DGEBA epoxy. This is due to the vitrification which only occurs in DGEBA epoxy, but not in DGEB epoxy. In DGEBA epoxy, the increase in fluorescence originates from the cure products alone rather than from the viscosity changes. Thus the total fluorescence intensity can be written as I_(F) =cΣF_(i) A_(i) where F_(i) is the fluorescence intensity ratio obtained for each cure product under the same experimetal conditions, A_(i) is their concentration and c is the experimental constant. The fluorescence intensity ratio for the cross-linker model compound is found to be independent of cure. Using the concentration values obtained by deconvolution of UV-VIS spectra, shown in Table III, the predicted I_(F) agrees well with the experimental points. Much greater values of I_(F) for DGEB are thus a direct consequence of further cure reactions, as also indicated by UV-VIS results as well as by IR monitoring of epoxy ring disappearance.

The extent of cure reaction is estimated from the fluorescence intensity values. As described before, the total fluorescence intensity is attributed to by the fluorescence of each cure product.

    I.sub.F =c(F.sub.pp A.sub.pp +F.sub.ps A.sub.ps +F.sub.ss A.sub.ss +F.sub.pt A.sub.pt +F.sub.st A.sub.st +F.sub.tt A.sub.tt) (20)

From the model compound studies, the following fluorescence intensity ratio was estimated.

    F.sub.pp :F.sub.ps :F.sub.ss :F.sub.pt :F.sub.st :F.sub.tt =1:9:18:700:1100:1400                                     (21)

Thus

    i.sub.F =c(A.sub.pp +9A.sub.ps +18A.sub.ss +700A.sub.pt +1100A.sub.st +1400A.sub.tt)                                            (22)

Substituting A_(ps), A_(ss), A_(pt), A_(st) and A_(tt) by A_(pp) and the amine reactivity ratio, r, according to Eqns. (8-12), one obtains ##EQU5## Eqn. (22) shows that I_(F) is a function of only A_(pp) for a given r. Using r=1 as determined from the extensive data in the previous section, and the experimental constant c, estimated from a few sets of known compositions to be on the order of 0.01, App is determined for a given I_(F) using Eqn. (22).

Once A_(pp) values are calculated, the concentration of other cure products can be calculated according to Eqn. (8-12). FIG. 7 shows a graphical representation of the data in comparison to the results obtained by UV-VIS deconvolution. As can be seen, the cure composition obtained from fluorescence is in reasonable agreement with the data obtained by deconvolution of UV-visible spectra.

In order to correlate fluorescence intensity at 565 nm with the overall extent of amine reaction, I_(F) versus ε_(a) is plotted for both epoxies cured at three isothermal cure temperatures, 140° C., 160° C., and 180° C., shown in FIG. 8. In this figure, ε_(a) is estimated from deconvolution of UV-visible spectra. All the data fall on a single smooth curve whose slope is much sharper at later stages of cure, especially after gelation. In other words, this fluorescence is a very sensitive monitoring technique for cure beyond gelation, because its intensity comes mostly from tertiary amine products.

5. Molecular Weight and Soluble Fractions as a Function of Cure

By extending a recursive approach developed for ideal networks, Miller and Macosko in Macromolecules 13, 1063 (1980) derived molecular average properties of polymer networks in systems with first-shell substitution effects. This applies in the epoxy-diamine system where the primary amine may have a different reactivity as compared with the secondary amine.

According to their theory, the weight average molecular weight, M_(w) can be written for a stoichiometric mixture of epoxy-diamine as follows: ##EQU6## where ε_(a) is the overall extent of cure reaction as defined by Eqn. (1), M_(A) and M_(B) are the molecular weight of the diamine and the diepoxide, respectively and μ is the weight average extent of reaction as defined by: ##EQU7## Using M_(A) =212 (DAA) and M_(B) =202 (DGEB) or 340 (DGEBA), the molecular weight and soluble fractions as a function of cure can be determined. According to Eq. (8-12), expected values of A_(ps), A_(ss), A_(pt), A_(st) and A_(tt) can be generated as a function A_(pp) if r is known. For a given r value (for example, 1 or 1.5), a ε_(a), according to Eqn. 1, and a corresponding μ value, according to Eqn. 25, is obtained from each set of expected values of cure products.

These values (ε_(a) and μ) are substituted in Eqn. 23 to generate theoretical predictions of M_(w) as a function of ε_(a). M_(w) does not change much whether it is 1 or 1.5, as shown by the closeness of two solid lines.

One can determine concentration of each of the cure products by comparing these theoretical predictions with the experimental data from the UV-VIS curve deconvolution assuming at the early stages of cure, before gelation, only one side of diepoxide is reacted with diamine, leaving the other side unreacted in all cure products. This is a reasonable assumption when there are many unreacted diamines present, thus making molecular weight calculation of the cure species easy. In order to experimentally determine the molecular weight of the whole system, one needs to know the molecular weight of each cure species. With the molecular weight values of cure species (e.g., A_(st) =M_(A) +3M_(B)) and compositional analyses (Table III), M_(w) of the epoxy system as a function of ε_(a) is easily estimated. The experimental values closely follow the theoretical prediction in the early stages of cure for both DGEBA and DGEB epoxy. However, as the cure approaches gelation, experimental points are much smaller than predicted, since the assumption is not valid at higher conversion.

Up to the gel point, all molecules are finite and therefore soluble. After the gel point, the weight soluble fraction, W_(s), drops sharply. At 100% reaction, one expects to have an infinite network with no soluble species. Theoretical curves are derived as described in Miller & Macosko's paper in Macromolecules, 9, 206 (1976) and extended by Charlesworth in J. Polym. Sci. Polym. Phys., 17, 1557 (1979). Part of the derivation as it applies to the present system is shown below:

    W.sub.s =W.sub.Af P(F.sub.A.sup.out).sup.4 +W.sub.B P(F.sub.B.sup.out).sup.2                                  (25)

where

W_(AF) =wt. fraction of amine molecules

W_(B) =wt. fraction of epoxy molecules

P(F_(A) ^(out))=probability that, looking out from an amine molecule, leads to a finite or dangling chain

P(F_(B) ^(out))=probability that, looking out from an epoxy molecule, leads to a finite or dangling chain

P(F_(A) ^(out)) and P(F_(B) ^(out)) can be expressed in terms of the extent of reaction, ε_(a), for f=4: ##EQU8## P(F_(B) ^(out)) is then defined as: ##EQU9## substituting Eqn. (26) and (27), ##EQU10## The dotted line in FIG. 9 shows the predicted weight fraction soluble, W_(s) as a function of ε_(a), according to Eqn. (28). Some cure product compositions are also indicated in FIG. 9. At high cure conversion (ε_(a) >0.8 for example), W_(s) is only comparable to the unreacted DAA concentration (A_(pp)). This is quite reasonable since other cure products are expected to be incorporated into the network. At somewhat lower conversion (ε_(a) approximately 0.7), the soluble fraction seems close to the added fractions of unreacted DAA (A_(pp)) and A_(ps), the first product of the cure. This result illustrates estimating the soluble fraction from cure product composition rather than from tedious and destructive solvent extraction studies.

6. IR Studies and Comparison with UV-VIS Studies

IR spectroscopy can be used to monitor the cure from the epoxide functionality simultaneously with UV-VIS and fluorescence studies following the cure reactions from the amine functionality of DAA. The extent of the epoxy reaction, ε_(b) for DGEBA-DDS epxoy, as defined by the following equation, ##EQU11## where A₉₁₅ cm⁻¹ is the IR absorption due to epoxy ring and A₁₁₈₄ cm⁻¹ is the absorption due to C--C stretching of the bridge carbon atom in DGEBA can be plotted. For DGEB-DDS, A₁₆₁₀ cm⁻¹ is used to calibrate for thickness fluctuation during cure, due to the phenyl ring in DDS, instead of A₁₁₈₄ cm⁻¹. The curves are s-shaped, showing strong cure temperature effects, especially in the early stages of cure. The extent of epoxy cure, ε_(b) reaches about 80% maximum in DGEBA-DDS epoxy, and about 95% at 180° C. cure in DGEB-DDS epoxy. In DGEBA-DDS epoxy, a leveling-off at high conversion due to vitrification is observed. When compared at the same temperature, 160° C., greater cure is observed in DGEB-DDS than in DGEBA-DDS which is consistent with UV-VIS and fluorescence results.

In order to compare the extent of amine reactions of DAA and epoxide reactions, ε_(a) by UV-VIS deconvolution versus ε_(b) for three cure temperatures for DGEB-DDS epoxy, where the reaction is almost complete, is plotted. The straight line shows the slope of one which corresponds to the ε_(a) =ε_(b) case. The data in general indicate that ε_(a) is somewhat greater than ε_(b) meaning faster consumption of DAA amine than overall epoxide. Since most of the epoxide reacts with DDS, this type of behavior is an indication that DAA may react a little faster than DDS. In order to confirm such differences in the reactivity between DDS and DDA, a scanning differential thermogram of a stoichiometric mixture of DGEB-DDS is compared with that of DGEB-DAA. Even though the areas under the exotherm are not very different, DGEB-DAA epoxy has an exotherm at a lower temperature, about 20° lower, indicating a faster cure reaction by DAA than DDS. Therefore, a calibration curve should be used in order to correlate the extent of reaction monitored by DAA with the extent of overall reaction in DDS containing epoxy.

EXAMPLE OF MEASURING THE EXTENT OF CURE BY DGEB-MDA LABELLED WITH DAS

p,p'-diamino stilbene (DAS) is used as a reactive label in another important epoxy amine resin, DGEB-MDS. DAS is selected since it is expected to have a reactivity similar to the curing agent, methylene dianiline (MDA), according to the Hammet constant reported by O. Exner in "A Critical Compilation at Substitute Constants" in Correlation Analysis in Chemistry by N. B. Chapman and J. Shorter, p.457 (Plenum Press, NY 1978). Unlike DAA, DAS is highly fluorescent even when the amine groups are primary, providing a greater sensitivity at early stages of cure. The photophysical behavior of trans stilbene and some of its derivatives are known to be sensitive to environmental viscosity. Thus, one can expect to see the effects on photophysical behavior from both the increasing viscosity and the chemical reaction as cure proceeds. DGEB was chosen since the cure reaction can progress much further at cure temperatures above 80° C. due to the absence of vitrification.

DGEB (98% purity) was purchased from Aldrich and used without further purification. MDA was recrystallized from toluene. DAS-dihydrochloride from Aldrich was neutralized with sodium carbonate and recrystallized from methanol to obtain the free amine.

A stoichiometric mixture of DGEB and MDA was used with 0.1% of DAS as a label. Sample preparation method was the same as in the previous example. A Perkin-Elmer UV-VIS Diode Array (Model 3840) and Fluorescence Spectrometer (MPF-66) with a Model 7500 Data Station were used to record the spectra. UV-VIS and fluorescence spectra were obtained after cooling the sample to room temperature following cure in an oven for a specific time. Fully substituted DAS (tt-DAS) was synthesized by reacting DAS with a large excess of phenyl glycidyl ether (PGE) at 140° C. for 12 hrs., followed by removal of the excess PGE under vacuum. A UV-VIS absorption peak at 372 nm was observed for tt-Das. The extent of reaction based on epoxy ring disappearance was estimated by monitoring IR absorption at 915 cm⁻¹ (epoxy ring) corrected with the absorption at 1610 cm⁻¹ (MDA internal standard).

1. Uv-visible absorption spectra

FIG. 10h shows the UV-VIS spectra of DGEB-MDA-DAS (0.1%) following cure at 140° C. Before cure, DAS absorption maxima is at 352 nm. DAS in DGEB shows another absorption peak at 327 nm, which is hidden under MDA absorption in DGEB-MDA epoxy. As cure proceeds, red shifts of the peak at 352 nm occur. After 300 minutes of cure at 140° C., the DAS absorption peak is shifted to 371 nm. Further curing, even at high temperatures, does not increase this absorption beyond 372 nm.

Red shifts in DAA and DAS are caused by the electron donating effect of the tertiary amines as compared to the primary amines. Assuming that such electronic effects are of similar magnitude for DAA and DAS, one would expect smaller red shifts in DAS compared to DAA. This is due to the fact that it requires greater energy to produce the same shifts in the uv region where DAS absorbs as compared to the visible region where DAA absorbs. Due to such small shifts in DAS labelled epoxy, it is not possible to deconvolute the absorption spectra.

2. Fluorescence Spectra

FIG. 10b shows fluorescence spectra as a function of cure time at 140° C. At zero cure time, the emission maximum is at 418 nm and a shoulder at 403 nm. As cure time increases, enhancement of emission as well as splitting into two peaks (at 418 nm and 430 nm) occur, as shown in FIG. 10b. The emission spectra of tt-DAS in dilute solution of PGE shows the same two peaks, apparently red shifted as in UV-VIS spectra. The ratio of fluorescence quantum yield of tt-DAS to DAS in dilute solution is found to be about 2.4 at 418 nm.

FIG. 11 shows the s-shaped curves of the emission intensity at 418 nm as a function of cure time at two cure temperatures, 140° and 120° C., respectively. At these temperatures, the gel time for DGEBA-MDA epoxy is known to be about 8 min. and 16 min., respectively. One may assume that the gel times are similar for DGEB-MDA epoxy. After gelation, fluorescence emission increases sharply at the respective curve temperature. Also, it is noted that the overall increase in emission intensity is usually about 3 to 3.5 times greater as compared to zero cure time at longer cure times.

Emission intensity of tt-DAS in DGEB-MDA epoxy is relatively constant until the later stages of cure. From this behavior, one can conclude that the chemical reaction, substitution on DAS amine groups, is the main cause for the emission enhancement near gelation but that the viscosity effect begins to contribute to the enhancement of the emission at later stages of cure.

A plot of fluorescence intensity at 418 mn as a function of cure based on the epoxy ring disappearance shows that the fluorescence intensity increases sharply after gelation, thus allowing for sensitive cure monitoring during the later stages of cure.

    __________________________________________________________________________     FILE:                                                                              SSDECONV FORTRAN UCONN CMS REL3 SL302 4/86                                 C   THIS PROGRAM DECONVOLUTES A COMPOSITE CURVE USING GAUSSIAN                                                                     SSD00010                   C   BUTION FUNCTIONS: CURVE USING ONLY 80 CH/LINE.  SSD00020                       DIMENSION Y(2000), P(10), S(10), XM(10), G(2000), A(10)                                                                        SSD00030                       DIMENSION JXY(10), XMEAN(10), R(10), PAR(10), XY(1000,4)                                                                       SSD00040                       F1=3.1415926                                    SSD00050                       XD=1.                                           SSD00060                       READ (01,7) XL,XU,XXL,XXU,YYL,YYU               SSD00070                       NX=1+(XU-XL)/XC                                 SSD00080                   C                                                   SSD00090                       DO 15 I=1,NX                                    SSD00100                       READ(01,7) Y(I)                                 SSD00110                   7   FORMAT(F5.0)                                    SSD00120                   15  CONTINUE                                        SSD00130                   C                                                   SSD00140                   10  PRINT *, ` ASSUME NUMBER OF COMPONENTS, N?`     SSD00150                       READ *, N                                       SSD00160                   C                                                   SSD00170                   17  PRINT *, ` WANT TO ASSUME S(I)=40. FOR ALL I? (YES=2/NO=1)`                                                                    SSD00180                       READ *, ITEM                                    SSD00190                       IF(ITEM.EQ.1) GO TO 30                          SSD00200                       DO 20 I=1,N                                     SSD00210                       S(I)=40.                                        SSD00220                   20  CONTINUE                                        SSD00230                   C                                                   SSD00240                       DO 25 I=1,N                                     SSD00250                       WRITE(6,27) I,I                                 SSD00260                   27  FORMAT(` ASSUME XMEAN(`,I1,`), P(`,I1,`).`)     SSD00270                       READ *, XMEAN(I),P(I)                           SSD00280                       XM(I)=1.+(XMEAN(I)-XL)/XD                       SSD00290                   25  CONTINUE                                        SSD00300                       DO 55 I=1,N                                     SSD00310                       WRITE(6,57) XMEAN(I), S(I), P(I)                SSD00320                   57  FORMAT(3F10.1)                                  SSD00330                   55  CONTINUE                                        SSD00340                       PRINT *, ` IS THAT ASSUMPTION REASONABLE ? (YES=2/NO=1)`                                                                       SSD00350                       READ *, ITEM                                    SSD00360                       IF(ITEM.EQ.1) GO TO 17                          SSD00370                       GO TO 43                                        SSD00380                   C                                                   SSD00390                   30  DO 40 I=1,N                                     SSD00400                       WRITE(6,31) I,I,I                               SSD00410                   31  FORMAT(` ASSUME XMEAN(`,I1,`), S(`,I1,`), P(`,I1,`).`)                                                                         SSD00420                       READ *, XMEAN(I), S(I), P(I)                    SSD00430                       XM(I)=1.+(XMEAN(I)-XL)/XD                       SSD00440                   40  CONTINUE                                        SSD00450                   C                                                   SSD00460                       DO 49 I=1,N                                     SSD00470                       WRITE(6,41) XMEAN(I), S(I), P(I)                SSD00480                   41  FORMAT(3F10.1)                                  SSD00490                   49  CONTINUE                                        SSD00500                       PRINT *, ` IS THAT ASSUMPTION REASONABLE ? (YES=2/NO=1)`                                                                       SSD00510                       READ *, ITEM                                    SSD00520                       IF(ITEM.EQ.1) GO TO 17                          SSD00530                   C                                                   SSD00540                   43  DO 50 I=1,NX                                    SSD00550                   FILE:                                                                              SSDECONV FORTRAN A UCONN CMS REL3 SL302 4/86                                   FTEM=0.                                         SSD00560                       DO 45 J=1,N                                     SSD00570                       XTEM=FLOAT(I)                                   SSD00580                       FTEM=FTEM+F(XTEM,P(J),S(J),XM(J))               SSD00590                   45  CONTINUE                                        SSD00600                       G(I)=FTEM                                       SSD00610                   50  CONTINUE                                        SSD00620                       PRINT *, ` WANT GRAPH? (YES=2/NO=1)`            SSD00630                       READ *, ITEM                                    SSD00640                       IF(ITEM.EQ.1) GO TO 67                          SSD00650                   C                                                   SSD00660                       NPLOT=500                                       SSD00670                       NAUTO=1                                         SSD00680                   60  DO 61 I=1,NX                                    SSD00690                       XY(I,1)=XL+XD*(I-1.)                            SSD00700                       XY(I,2)=Y(I)                                    SSD00710                       XY(I,3)=XY(I,1)                                 SSD00720                       XY(I,4)=G(I)                                    SSD00730                   61  CONTINUE                                        SSD00740                       DO 62 J=1,4                                     SSD00750                       JXY(J)=J                                        SSD00760                   62  CONTINUE                                        SSD00770                   65  CALL VPLOT(XY,JXY,NPLOT,1000,2,NAUTO,XXL,XXU,YYL,YYU)                                                                          SSD00780                       PRINT *, ` WANT PREVIOUS GRAPH AGAIN? (YES=2/NO=1)`                                                                            SSD00790                       READ *, ITEM                                    SSD00800                       IF(ITEM.EQ.2) GO TO 65                          SSD00810                   C                                                   SSD00820                   67  PRINT *, ` WANT ERROR ESTIMATION? (YES=2/NO=1)  SSD00830                       READ *, ITEM                                    SSD00840                       IF(ITEM.EQ.1) GO TO 90                          SSD00850                       PRINT *, ` CALCULATE ERROR FROM XLL TO XUU.`    SSD00860                       READ *, XLL,XUU                                 SSD00870                       NXLL=(XLL-XL)/XD+1                              SSD00880                       NXUU=(XUU-XL)/XD+1                              SSD00890                   70  ATEM=0.                                         SSD00900                       BTEM=0.                                         SSD00910                       DO 80 I=NXLL,NXUU                               SSD00920                       ATEM=ATEM+(Y(I)-G(I))**2                        SSD00930                       BTEM=BTEM+Y(I)                                  SSD00940                   80  CONTINUE                                        SSD00950                       STDV=100.*(ATEM**(0.5))/BTEM                    SSD00960                       WRITE(6,81) XLL,XUU,STDV                        SSD00970                   81  FORMAT(` ERROR OF ESTIMATION FROM (`, F5.1 ,`) TO (`, F5.1 ,`)                                                                 SSD00980                       $ `, F7.3, ` %`)                                SSD00990                   C                                                   SSD01000                       PRINT *, ` WANT GRAPH ONE MORE TIME? (YES=2/NO=1)`                                                                             SSD01010                       READ *, ITEM                                    SSD01020                       IF(ITEM.EQ.1) GO TO 90                          SSD01030                       CALL VPLOT(XY,JXY,NPLOT,1000,2,NAUTO,XXL,XXU,YYL,YYU)                                                                          SSD01040                   90  DO 100 I=1,N                                    SSD01050                       A(I)=(P(I)*S(I))*((PI*2.)**0.5)                 SSD01060                       WRITE(6,91) I,XMEAN(I),I,S(I),I,P(I),I,A(I)     SSD01070                   91  FORMAT(` XMEAN(`,I1,`) = `, F6.1,2X,` S(`,I1,`) = `, F5.1,2X,`                                                                 SSD01080                       $(`,I1,`)= `, F7.1,2X,`; AREA(`,I1,`) = `, F12.1)                                                                              SSD01090                   100 CONTINUE                                        SSD01100                   FILE:                                                                              SSDECONV FORTRAN A UCONN CMS REL3 SL302 4/86                               C                                                   SSD01110                       AR=0.                                           SSD01120                       DO 99 I=1,N                                     SSD01130                       AR=AR+A(I)                                      SSD01140                   99  CONTINUE                                        SSD01150                       DO 96 I=1,N                                     SSD01160                       R(I)=A(I)/A(1)                                  SSD01170                       PAR(I)=100.*A(I)/AR                             SSD01180                       WRITE (6,94) I,R(I),I,PAR(I)                    SSD01190                   94  FORMAT(/` R(`,I1,`)= `, F5.2, 5X, `PAR(`,I1,`)= `, F7.2,                                                                       SSD01200                   96  CONTINUE                                        SSD01210                   C                                                   SSD01220                       PRINT *, ` WANT TO REPEAT? (YES=2/NO=1)`        SSD01230                       READ *, ITEM                                    SSD01240                       IF(ITEM.EQ.2) GO TO 10                          SSD01250                       STOP                                            SSD01260                       END                                             SSD01270                   C                                                   SSD01280                       FUNCTION F(X,P,S,XM)                            SSD01290                       XTEM=(X-XM)**2/(2.*S**2)                        SSD01300                       F=0.                                            SSD01310                       IF(XTEM.LT.100) F=P/EXP(XTEM)                   SSD01320                       RETURN                                          SSD01330                       END                                             SSD01340                   C                                                   SSD01350                       SUBROUTINE VPLOT(XY,JXY,N,NDIM,NCUR,ISCALE,XL,XU,YL,YU)                                                                        SSD01360                       DIMENSION IGRID(61),XS(11),YS(13),ICHAR(7),XY(5000),JXY(10)                                                                    SSD01370                       DATA ICHAR/1H+,1H*,1H-,1H$,1H=,1H.,1H /         SSD01380                       XS(1)=XL                                        SSD01390                       XMAX=XU                                         SSD01400                       YMIN=YL                                         SSD01410                       YS(1)=YU                                        SSD01420                       IF(ISCALE.NE.0) GO TO 36                        SSD01430                       XMAX=-1.0E 20                                   SSD01440                       XS(1)=-XMAX                                     SSD01450                       YS(1)=XMAX                                      SSD01460                       YMIN=XS(1)                                      SSD01470                       J2=0                                            SSD01480                       DO 35 J=1,NCUR                                  SSD01490                       J2=J2+2                                         SSD01500                       JIX=(JXY(J2-1)-1)*NDIM                          SSD01510                       JIY=(JXY(J2)-1)*NDIM                            SSD01520                       DO 35 I=1,N                                     SSD01530                       IJX=JIX+I                                       SSD01540                       IJY=JIY+I                                       SSD01550                       IF(XY(IJX).GT.XMAX)XMAX=XY(IJX)                 SSD01560                       IF(XY(IJX).LT.XS(1))XS(1)=XY(IJX)               SSD01570                       IF(XY(IJY).GT.YS(1))YS(1)=XY(IJY)               SSD01580                       IF(XY(IJY).LT.YMIN)YMIN=XY(IJY)                 SSD01590                   35  CONTINUE                                        SSD01600                   36  XR=XMAX-XS(1)                                   SSD01610                       IF(XR.EQ.C.0)XR=1.0E-20                         SSD01620                       YR=YS(1)-YMIN                                   SSD01630                       IF(YR.EQ.0.0)YR=1.0E-20                         SSD01640                       XT=XMAX*XS(1)                                   SSD01650                   FILE:                                                                              SSDECONV FORTRAN A UCONN CMS REL3 SL302 4/86                                   YT=YMIN*YS(1)                                   SSD01660                       IF(XT.LT.0.0)IYAX=60.0*(-XS(1))/XR+1.5          SSD01670                       IF(YT.LE.0.0)IXAX=48.0*YS(1)/YR+1.5             SSD01680                       XMAX=XR/10.                                     SSD01690                       DO 46 I=2,11                                    SSD01700                   46  XS(I)=XS(I-1)+XMAX                              SSD01710                       XMAX=YR/12.                                     SSD01720                       DO 47 I=2,13                                    SSD01730                   47  YS(I)=YS(I-1)-XMAX                              SSD01740                       WRITE(6,71) (XS(I),I=1,11)                      SSD01750                       WRITE(3,71) (XS(I),I=1,11)                      SSD01760                       II=1                                            SSD01770                       KK=0                                            SSD01780                       DO 146 LINE=1,49                                SSD01790                       DO 104 J=1,61                                   SSD01800                   104 IGRID(J)=ICHAR(7)                               SSD01810                       IF(YT.GT.0.0) GO TO 109                         SSD01820                       IF(LINE.NE.IXAX) GO TO 109                      SSD01830                       DO 105 J=1,61                                   SSD01840                   105 IGRID(J)=ICHAR(6)                               SSD01850                   109 IF(XT.LT.0.0) IGRID(IYAX)=ICHAR(6)              SSD01860                       J2=0                                            SSD01870                       DO 125 J=1,NCUR                                 SSD01880                       J2=J2+2                                         SSD01890                       JIX=(JXY(J2-1)-1)*NDIM                          SSD01900                       JIY=(JXY(J2)-1)*NDIM                            SSD01910                       JC=MOD(J,5)                                     SSD01920                       DO 125 I=1,N                                    SSD01930                       IJX=JIX+I                                       SSD01940                       IJY=JIY+I                                       SSD01950                       IPTY=48.0*(YS(1)-XY(IJY))/YR+1.5                SSD01960                       IF(IPTY.GT.49)IPTY=49                           SSD01970                       IF(IPTY.LT.1)IPTY=1                             SSD01980                       IF(IPTY.NE.LINE) GO TO 125                      SSD01990                       IPTX=60.0*(XY(IJX)-XS(1))/XR+1.5                SSD02000                       IF(IPTX.LT.1)IPTX=1                             SSD02010                       IF(IPTX.GT.61)IPTX=61                           SSD02020                       IF(JC.NE.0) GO TO 119                           SSD02030                       IGRID(IPTX)=ICHAR(5)                            SSD02040                       GO TO 125                                       SSD02050                   119 IGRID(IPTX)= ICHAR(JC)                          SSD02060                   125 CONTINUE                                        SSD02070                       IF(KK.GT.0)GO TO 134                            SSD02080                       WRITE(6,72) YS(II),(IGRID(I), I=1,61),YS(II)    SSD02090                       WRITE(3,72) YS(II),(IGRID(I), I=1,61),YS(II)    SSD02100                       II=II+1                                         SSD02110                       GO TO 135                                       SSD02120                   134 WRITE(6,73) (IGRID(I), I=1,61)                  SSD02130                       WRITE(3,73) (IGRID(I), I=1,61)                  SSD02140                   135 KK=KK+1                                         SSD02150                       IF(KK.NE.4) GO TO 146                           SSD02160                       KK=0                                            SSD02170                   146 CONTINUE                                        SSD02180                       WRITE(6,74) (XS(I), I=1,11)                     SSD02190                       WRITE(3,74) (XS(I), I=1,11)                     SSD02200                   FILE:                                                                              SSDECONV FORTRAN A UCONN CMS REL3 SL302 4/86                               71  FORMAT(F9.0,10F6.0/6X,1H*,20(3H+**),2H+*)       SSD02210                   72  FORMAT(F5.0,1X,1H+,61A1,1H+,1X,F5.0)            SSD02220                   73  FORMAT(6X,1H*,61A1,1H*)                         SSD02230                   74  FORMAT(6X,1H*,20(3H+**),2H+*/F9.0,10F6.0)       SSD02240                       RETURN                                          SSD02250                       END                                             SSD02260                   __________________________________________________________________________

The present invention may be embodied in other specific forms without departing from the spirit and scope thereof. These and other modifications of the invention will occur to those skilled in the art and are intended to fall within the scope of the appended claims. 

I claim:
 1. An epoxy system comprising epoxide, a curing agent, and a label selected from the group consisting of azonapthalene dyes, benzylidene-dianiline dyes and conjugated stilbene dyes, wherein said label reacts with the epoxide at a rate comparable to the reaction rate of the curing agent with the epoxide, said label and concentration of said label selected to produce a fluorescence intensity proportional to the concentration of epoxy cure products.
 2. The epoxy system of claim 1 comprising p,p'-diaminostilbene.
 3. A method for determining the extent of polymerization of an epoxy system including epoxide and a curing agent comprisingadding a label to the epoxy system, wherein said label is selected from the group consisting of azonapthalene dyes, benzylidene-dianiline dyes, and conjugated stilbene dyes and said label reacts with the epoxide at a rate comparable to the reaction rate of the curing agent with the epoxide, said label being selected and added at a concentration producing a fluorescence intensity proportional to the concentration of epoxy cure products, polymerizing the epoxy system, measuring the fluorescence of the polymerizing epoxy system, and comparing the measured fluorescence with a curve relating fluorescence intensity to the extent of polymerization.
 4. The method of claim 3 further comprising determining the fluorescence intensity per mole of the epoxy compound when excited at the absorption maximum.
 5. The method of claim 3 further comprising correcting for different reactivity rates of the label and the curing agent.
 6. The method of claim 3 further comprising determining the reactivity ratio of the primary amine to secondary amine in the epoxy system.
 7. The method of claim 6 further comprising determining the reaction rate constant.
 8. The method of claim 7 further comprising determining the concentration of initial, intermediate and final polymerization products.
 9. The method of claim 8 further comprising determining the soluble weight fraction from the concentration of the initial, intermediate and final polymerization products.
 10. The method of claim 3 further comprising measuring the UV-VIS spectra of the polymerizing epoxy system.
 11. The method of claim 10 wherein said UV-VIS absorption spectra is measured.
 12. The method of claim 10 wherein said UV-VIS excitation spectra is measured.
 13. The method of claim 10 further comprising deconvoluting the UV-VIS spectra to determine the concentration of the initial, intermediate and final polymerization products.
 14. The method of claim 13 further comprising calibrating for concentration and thickness changes during polymerization using the UV-VIS spectra for said label.
 15. The method of claim 13, further comprising determining the ratios of cross linker:branch point:linear-chain:chain-end:label.
 16. The method of claim 13, further comprising modeling the overall fluorescence intensity as the sum of the fluorescence intensity of the initial, intermediate and final polymerization products.
 17. The method of claim 16 further comprising constructing a curve relating fluorescence intensity with the extent of polymerization.
 18. The method of claim 13 further comprising determining the soluble weight fraction from the concentration of the initial, intermediate and final polymerization products.
 19. An epoxy system comprising epoxide, a curing agent, and an azobenzene dye at a concentration of up to about 0.01% by weight, wherein said azobenzene dye reacts with the epoxide at a rate comparable to the reaction rate of the curing agent with the epoxide said label being selected and added at a concentration producing a fluorescence intensity proportional to the concentration of epoxy cure product.
 20. A method for determining the extent of polymerization of an epoxy system including epoxide and a curing agent comprisingadding an azobenzene dye to the epoxy system to a concentration of up to about 0.01% by weight, said label reacting with the epoxide at a rate comparable to the reaction rate of the curing agent with the epoxide said label being selected and added at a concentration producing a fluorescence intensity proportional to the concentration of epoxy cure product, polymerizing the epoxy system, measuring the fluorescence of the polymerizing epoxy system, and comparing the measured fluorescence with a curve relating fluorescence intensity to the extent of polymerization.
 21. The method of claim 20 further comprising correcting for different reactivity rates of the label and the curing agent.
 22. The method of claim 20 further comprising determining the reactivity ratio of the primary amine to secondary amine in the epoxy system.
 23. The method of claim 22 further comprising determining the reaction rate constant.
 24. The method of claim 23 further comprising determining the concentration of initial, intermediate and final polymerization products.
 25. The method of claim 24 further comprising determining the soluble weight fraction from the concentration of the initial, intermediate and final polymerization products. 